This invention relates to a process for recovering or isolating proteins and polymers in solid form from liquids containing the same. More particularly, it relates to a process for recovering or isolating proteins and polymers from solutions, dispersions and latices of the proteins or polymers, in a controllable particle size, by a desolventizing technique or by treatment with a coagulating agent.
Proteins
Proteins arise from numerous sources and are known to be important for the proper nutrition of humans and domestic animals. Proteins also find use as soil nutrients and are widely used as fertilizers. To place the protein in a useful form, it is often necessary to extract the proteins from a suitable source material by means of a fluid, usually water. The extracted protein is dissolved or suspended in the fluid. The fluid is then heated and/or treated with various known chemicals which cause the protein in the fluid to coagulate. The system then separates into a two-phase system comprising a curd of coagulated protein and carrier fluid. The coagulated protein is separated from the fluid and dried to yield a protein product that can be conveniently handled, shipped and compounded.
One important commercial source of protein is animal blood, such as beef or pork blood which is available in large quantity and at relatively low cost from the slaughterhouses. In animal blood, the protein content is suspended in a fluid called plasma. The recovery of proteins from animal blood is of considerable commercial significance and consequently, the protein aspects of the invention are discussed primarily in terms of the coagulation of protein from whole or diluted animal blood. However, it is to be understood that the invention can be advantageously applied to the coagulation of various other coagulable proteins and that the discussion in terms of blood protein is for the purpose of illustration only.
Blood is valued for its high protein content and is a common additive in fertilizers and animal feeds. To facilitate its use in such applications, the protein is coagulated so it can be separated from the rest of the blood as described above. The separated protein is then dried to eliminate moisture which can cause the protein product to spoil. A dried product is also more convenient to handle and ship.
The art of coagulating animal blood is relatively old. In one prior process, the blood was thermally coagulated by mixing it with a hot fluid such as steam. Blood has also been coagulated by treatment with one or more of numerous chemical coagulants, including inorganic salts, strong electrolytes such as sulfuric acid, acetic acid, hydrochloric acid, and strong ionizable bases such as sodium hydroxide and lithium hydroxide. Factors such as the type and relative amounts of the coagulating agent used, and the time and temperature of the coagulation process can have a significant influence on the quality of the coagulated protein. With less severe coagulating treatments, less protein molecules are normally destroyed and hence a higher quality product is obtained. When more severe coagulating treatments are used, more protein molecules are destroyed and a lower quality product results. The higher quality protein is used as an animal feed, while the lower quality protein is used as a fertilizer.
The majority of the prior art blood coagulation processes have been batch operations. In a typical process, steam was bubbled into a tank of blood. The heat and mass transfer in such a process was usually poor. Furthermore, the blood often coagulated in such a manner that it enclosed pockets of uncoagulated blood which because of the poor heat transfer properties of the outer coagulated matter, required long residence times to complete the coagulation. The tank was emptied after the coagulation was judged to be complete and a new batch of blood was then added.
To overcome some of the problems related to batch processes for coagulating blood, several continuous processes for coagulating blood have been proposed. In one process, blood was pumped through a perforated tube and steam was injected into the blood through the perforations to cause coagulation. The perforated tube was concentric with a tube which extended downstream of the perforated tube. The length of this latter tube was chosen so that the coagulation was completed at the exit end of the tube. The coagulated product was then centrifuged for partial removal of the liquid and the centrifuge cake was dried. One disadvantage of this process was the tendency of the perforations and the tubing to become blocked with coagulated blood. This can seriously impair heat transfer from the steam to the blood, resulting in reduced coagulation capacity. Furthermore, deposition of coagulated matter on the walls of the tubing could cause blockages necessitating a shut down of the equipment to clean out the deposits and get the feed stream flowing continuously once more. Another problem often encountered with this process was non-uniform coagulation of the product in which pockets of uncoagulated blood were enclosed by coagulated matter. In the subsequent drying step, the product dried on the outside but the inside still contained moisture which could cause the product to putrefy during storage. To guard against this problem of incomplete coagulation, the blood was often pretreated with chemical coagulants in order to at least partially precoagulate the blood, thereby making it easier to achieve a complete coagulation when the steam was added. However, these chemical pretreatments are usually costly and difficult to maintain in actual practice and are, therefore, undesirable.
A process characterized by poor heat transfer is also not generally suitable for the coagulation of fresh blood. Blood is usually considered fresh when it has not yet begun to coagulate to any substantial extent. In most cases it is untreated blood three hours or less out of the animal. When the coagulation process takes place under poor conditions of heat transfer, it is often necessary to wait until the blood has partially begun to coagulate before it can be processed, so that less of a demand is placed on the coagulation system itself. Another disadvantage of a poor heat transfer process is that the blood is exposed to heating for prolonged periods of time which can cause degradation of the blood proteins. If thermally degraded blood protein was used as an animal feed supplement, it would be less nutritious to the animal than blood proteins which, during processing, were exposed to heat for only short periods of time.
In another prior art continuous coagulation process, blood was fed to a screw conveyor which moved it past a perforated area where steam injection took place. This method has many of the same disadvantages discussed above for the perforated tube coagulation process.
The coagulation processes of the prior art are generally characterized by relatively inefficient heat and mass transfer conditions. Because of this, it takes considerable time to transfer heat from its point of introduction to blood at some distant point and to bring the blood to the temperature at which cogulation is initiated. Moreover, as the blook begins to coagulate, heat transfer is further impaired since the thermal conductivity of blood decreases as coagulation proceeds and the blood becomes more viscous and changes from a liquid into a solid. Similar considerations apply to mass transfer. If a chemical coagulating agent is used, the coagulating agent must be transferred to blood distant from its point of introduction. This transfer is impaired as the blood between the point of introduction of the agent and the blood under consideration coagulates. In summary, the blood coagulates in steps with the blood closest to the point of introduction of coagulating agent coagulating first and the blood further away from the point of introduction at some time later. This stepwise process requires a relatively long period of time from initiation to completion.
A further disadvantage of prior art coagulation processes is the inefficiency of these processes in coagulating protein or blood which has been diluted with water or other diluents. Diluted blood is difficult to coagulate thermally because the diluent as well as the blood must be heated to coagulating temperatures. Furthermore, since diluted blood has a lower protein concentration, it is often difficult to coagulate the blood protein to a particle size which can be readily separated from the carrier liquid.
It is therefore an object of this invention to provide a process for coagulating, agglomerating or otherwise recovering proteins dissolved or suspended in fluids which overcomes the disadvantages of the prior art processes.
It is another object of this invention to provide an efficient process for continuously coagulating the protein in fluids such as blood into a flowable slurry of finely divided, non-sticky, coagulated protein particles.
It is another object of this invention to provide an efficient process for continuously coagulating the protein in liquids such as blood in which uniformly coagulated protein particles are produced which can be uniformly dried.
It is another object of this invention to provide a process for continuously coagulating the protein in blood and other materials in which the mass and/or heat transfer are highly efficient so that the protein is coagulated substantially instantaneously with minimal retention time and minimum heating requirements.
It is another object of this invention to provide a process for continuously coagulating the protein of fresh or aged blood, and diluted or undiluted blood.
Polymers
Coagulation also plays an important role in the recovery of many natural and synthetic polymers from liquids which contain the polymers. For example, many commercially important polymers and copolymers are produced by emulsion polymerization of the precursor monomer or monomers. Illustrative polymers manufactured by this technique include polychloroprene (neoprene), styrene-butadiene rubber, polybutadiene, polyethylene, polypropylene, acrylonitrile-butadiene copolymer, and many others known to those skilled in the art and reported in the literature. In an emulsion polymerization process, the monomer is first emulsified in an aqueous medium with an appropriate reaction catalyst. The emulsifying agent used produces a stable emulsion of the monomer or monomers in the aqueous phase. Various additives dissolved in the monomer initiate, direct and finally terminate the polymerization process, in accordance with known procedures. The ultimate product of the emulsion polymerization process is a stable colloidal dispersion of finely sized polymer or copolymer in water called a "latex".
Under normal conditions of storage and handling, a stable latex may be defined as one in which substantially no coalescence or agglomeration of the polymer particles occurs. A microscopic picture of a typical latex shows many small particles of polymers illustratively ranging from about 0.1 to 5 or more microns average diameter. Many latices have particle sizes of about 0.1 to 0.5 microns. For example, the particle size of various commercial neoprene latices is reported as varying between about 0.11 and 0.15 microns. Each discrete polymer particle is surrounded by a substantially monomolecular layer of emulsifier molecules.
Latex particles have an affinity for water, i.e., they are hydrophilic, and thus tend to attract and hold a sheath of water firmly around themselves. The colloidal latex particles also possess electrical properties that influence their behavior. For example, electrical charges on the particle surface establish an electrostatic field in which potential differences are largely due to concentration differences between the cationic and anionic species on the particle surfaces. The electrical potential at the boundary plane, i.e., the plane which divides the portion of the liquid around the particle that moves with the particle from the portion which can move independently of the particle, is called the zeta potential.
The stability of the latex depends largely upon the balance of the various attraction and repulsion forces acting on the colloidal latex particles. The attraction forces are commonly called Van der Waals forces. The repulsion forces result from the zeta potential and the bound liquid which envelopes the colloid particle.
In many cases, it becomes desirable or necessary to separate and recover the polymer from the latex in an agglomerated or concentrated form. The prior art describes numerous physical and chemical processes for concentrating or breaking polymeric latices such as centrifuging, evaporation, freezing and the addition of acids, electrolytes and other latex coagulating agents. The general objective of most of these processes is to reduce the repulsion forces between the latex particles to the extent that the attraction forces prevail. As this occurs, the latex particles coalesce to form larger ones commonly called the "polymer crumb," "rubber crumb" or "crumb" which are more easily separated from the system. The separated particles are then normally conveyed to dewatering equipment such as screw presses or dewatering screens for ultimate recovery of the dried or dewatered polymer.
In general, it is desirable to coagulate a latex to a particle size which can be readily transported in the conveying system and handled in the dewatering equipment. If the coagulated particles are too small, they may not convey properly. They can also pass through the openings in dewatering screens and/or disappear with the effluent from the screw presses. On the other hand, if the coagulated particles are too large there may be conveying and handling problems of a different type, e.g., blockages, as well as insufficient drainage from the particles. The inclusion of uncoagulated material is also a serious problem with large particles. In addition, if there is excessive variation in particle size, it becomes difficult to uniformly dewater and dry the particles. These problems can be overcome or minimized if the latex particles are coagulated to a controlled particle size.
The prior art describes numerous techniques for coagulating latices. For example, many latices can be coagulated to a controlled paticle size by the addition of one or more electrolytes. See, for example, U.S. Pat. Nos. 2,366,460, 2,385,688, 2,386,449, 2,393,208, 2,393,348, 2,408,128, 2,459,748, 2,469,827, 2,476,822, 3,053,824 and 3,498,935. The electrolyte is believed to cause particles to coalesce by reducing the thickness of the layer of bound water on the latex particle and/or by reducing the zeta potential.
The coagulation usually takes place in a batch or continuous coagulator in which the latex and coagulant are mixed. The recovered polymer crumb is then washed to remove electrolyte remnants. The washing step is important since high levels of electrolyte or other contaminants in the crumb may adversely affect processability of the polymer or result in an unacceptable product.
In many cases, however, the addition of electrolyte to the latex produces large lumps or agglomerations of polymer which do not have a high enough surface area or porosity to permit effective washing out of the electrolyte. These lumps may also contain uncoagulated latex, and often cannot be readily conveyed or dewatered. The problem of lumping, for example, is particularly prevalent with polychloroprene (neoprene) latices which consist of relatively small hydrophilic particles having a strong negative charge. The polychloroprene latex particles exhibit vigorous Brownian movement which causes a rapid "chain-reaction" during coagulation, with the result that undesirably large agglomerates of coagulated polychloroprene tend to be formed. In short, polychloroprene coagulation is very sensitive and difficult to control.
To minimize the problems of uncontrolled coagulation of latex particles into large relatively unporous lumps, several processes have been proposed. For example, M. A. Youker, in his article "Continuous Isolation of GR-M From Latex," Chemical Engineering Progress ("Trans. Section"), Vol. 43, No. 8, pp. 391-398 (August 1947), describes a process in which a polychloroprene emulsion is broken by freezing the latex as a thin sheet. During the subsequent washing step, the frozen water in the sheet melts and leaves a porous structure which allows for effective washing of the sheet. One disadvantage of this technique is a low production rate compared to conventional coagulation techniques. See also British Pat. No. 876,283 which alludes to the difficulties of coagulating polychloroprene and natural rubber using electrolytes, and proposes to solve the problem by using as the coagulating agent an aqueous solution containing several different types of electrolytes.
Many commercially important polymers and copolymers are also made by a solution polymerization techniques as opposed to the emulsion polymerization technique just described. Illustrative polymers and copolymers manufactured by solution polymerization include polyisoprene, polybutadiene, natural balata rubber, styrene-butadiene rubber and others well known to those skilled in the art and described in the literature. In solution polymerization, the monomer or monomers and the additives which initiate, direct and terminate the polymerization are dissolved in a liquid, usually an organic liquid, in which the polymer product is soluble. The polymerization takes place in the liquid to produce a solution illustratively containing about 1 to 50% dissolved polymer. As with emulsions and latices, it often becomes necessary to recover and isolate the polymer or copolymer from the solution.
Various methods have been disclosed for recovering and isolating the dissolved polymer from such solutions. See, for example, U.S. Pat. Nos. 2,530,144, 2,561,256, 2,607,763, 2,833,750, 2,844,569, 2,957,855 and 2,957,861. In one common technique, the polymer solution is treated with a desolventizing agent, such as steam or hot water, which is substantially immiscible with the solution solvent and in which the polymer is substantially insoluble. The desolventizing agent heats the polymer solution to the point where the solvent volatilizes. As desolventizing occurs, the polymer comes out of solution in the form of solid particles which agglomerate into longer particles. The end result is a slurry consisting of polymer particles and liquid desolventizing agent, from which the polymer is recovered.
The desolventizing usually takes place in a continuous or batch process in which the polymer solution and desolventizing agent are mixed at atmospheric pressure or under vacuum. For economic reasons the vaporized solvent is normally condensed, recovered and recycled as make-up solvent to the polymerization process.
The problems discussed above with regard to the recovery and isolation of polymers from latices are also encountered in the recovery and isolation of polymers from solution by desolventizing. The formation of unduly large lumps or aggregates of polymer, for example, can be a serious problem, as discussed above. Thus the objective is to desolventize the polymer solution in such way as to yield a polymer of controlled particle size in order to facilitate handling, washing and drying of the polymer. In addition, the recovery of polymers from solution or from a latex is generally plagued by problems of heat and mass transfer similar to those discussed above in connection with the coagulation of blood. The coagulating and desolventizing agents must first reach the polymer particles or the bulk of the solvent before effective coagulation or desolventizing can occur. Heat transfer is a particular problem in the desolventizing of polymer solutions where the thermal energy carried by the desolventizing agent must transfer to the solvent once the desolventizing agent reaches the bulk of the solvent in order to initiate volatilization of the solvent. The high energy requirements of desolventizing are a significant drawback of such processes. A polymer recovery process which operated at low retention times and low energy requirements would be most desirable.
It is therefore also an object of this invention to provide a process for recovering a polymer dissolved or suspended in a liquid which overcomes the disadvantages of the prior art processes.
It is another object of this invention to provide an efficient process for continuously recovering or isolating a polymer from latices and solutions of the polymer in the form of a flowable slurry of polymer particles, by the use of desolventizing and coagulation techniques.
It is another object of this invention to provide an efficient process for continuously recovering a polymer from latices and solutions of the polymer in the form of uniformly coagulated particles which can be uniformly washed of contaminants to produce a final product of acceptable quality.
It is another object of this invention to provide a process for continuously recovering a polymer from latices and solutions of the polymer in which the mass and/or heat transfer are highly efficient so that the polymer is recovered substantially instantaneously with reduced retention time and reduced overall heating requirements, thereby providing a process having a practical and economical high rate of polymer recovery and production.
These and other objects of this invention will be apparent to those skilled in the art upon a consideration of this entire disclosure.